A density of a read only memory (defined "ROM" hereinafter) integrated circuit has been much increased in these days. In accordance with the increase of this integrated circuit density, means for separating memory elements from each other have been changing from selective oxidation separation to trench separation.
In a conventional process for providing the trench separation, field oxidation films are formed at predetermined regions on a p- semiconductor substrate by use of a selective oxidation method at a first stage. Although plural field oxidation films are provided on plural regions of the substrate, a limited section including only one field oxidation film will be explained hereinafter. At the next stage, a pad oxidation film having a thickness of 200 to 300 .ANG. is formed at the remaining region on the p- semiconductor substrate, and silicon nitride film is formed on the field and pad oxidation films. Thereafter, element separating trenches are provided on the p- semiconductor substrate by use of photoresists.
Next, a trench oxidation film is formed on the inner surface of the element separating trenches, and an oxidation film is then formed on the silicon nitride film and the trench oxidation films by providing an oxide having a high re-flow property such as BPSG, etc. After this, the high re-flow property oxidation film is re-flowed by a heat treatment of approximately 900.degree. to 1000.degree. C.
At the following stage, the re-flowed oxidation film is etched back to be left in the element separating trenches, so that the top surface of the film is above the top surface of the p- semiconductor substrate, and the silicon nitride film and the pad oxidation film are successively removed. Then, a gate oxidation film is formed on the p- semiconductor substrate thus processed, and a polycrystal silicon layer and a WSi layer are successively grown on the gate oxidation film.
Thereafter, a gate electrode is provided by defining the polycrystalline silicon layer and the WSi layer to be a predetermined pattern, and impurities are then injected to provide a source and a drain of a transistor, and then to provide a ROM code.
Finally, an interface insulation film is formed to be provided with contact holes, and aluminum wiring is provided in a predetermined pattern.
However, the conventional process for fabricating a semiconductor ROM has a disadvantage in that the gate oxidation film and a channel portion of a transistor are liable to be contaminated with phosphorus, because phosphorus contained in the element separating oxidation film is evaporated. The element separating oxidation film usually contains impurities such as phosphorus, etc. to provide a high re-flow property.
The conventional process for fabricating a semiconductor ROM has a further disadvantage in that the element separating oxidation film does not function as an element separating means, due to a short circuit between digit lines resulted from the occurence of an inverted layer on the sides of, and beneath the element separating trench, because the element separating oxidation film, for which BPSG, etc. is used to provide a sufficient re-flow property, is liable to be charged up by a positive potential, and the charged-up potential is increased in a practical use.